Versatile tracking arrays for a navigation system and methods of recovering registration using the same

ABSTRACT

A navigation system is disclosed comprising a first and a second tracker support separately affixed to the same rigid object by a distance. A first and a second plurality of trackable elements are secured to the first and second tracker supports, respectively. The navigation system defines a tracking arrangement to be tracked based on a combination of the first and second plurality of trackable elements. A geometry of the tracking arrangement relative to the rigid object is registered. The navigation system has a localizer configured to track the rigid object by detecting the registered geometry of the tracking arrangement. The navigation system identifies a condition wherein at least one trackable element has been displaced relative to the registered geometry, and in response, generates a response to address the condition.

CROSS-REFERENCE TO RELATED APPLICATIONS

The subject application claims the benefit of U.S. Provisional PatentApplication No. 62/676,497, filed May 25, 2018, and claims the benefitof U.S. Provisional Patent Application No. 62/792,147, filed Jan. 14,2019, the disclosures of which are hereby incorporated by reference intheir entirety.

TECHNICAL FIELD

The present disclosure relates generally to a system and method fortracking an object during a surgical procedure using a versatiletracking array defined by the combined geometry of separate trackersattached to the same object and techniques for recovering registrationof the combined geometry in the even that one of the trackers isdisplaced.

BACKGROUND

Navigation systems assist users in precisely locating objects. Forinstance, navigation systems are used in industrial, aerospace, andmedical applications. In the medical field, navigation systems assistsurgeons in precisely placing surgical instruments relative to a targetsite in a patient, for example, during a surgical operation. The targetsite usually requires some form of treatment, such as tissue removal.Conventional navigation systems employ a localizer including one or moreimaging technologies that cooperate with trackers to provide positionand/or orientation data associated with the surgical instrument and thetarget site, e.g., the volume of bone to be removed. These trackersallow a surgeon to see the position and/or orientation of the surgicaltool overlaid on a monitor in conjunction with a preoperative or anintraoperative image of the patient. The preoperative images may begenerated by MRI, CT scans, or other well-known medical imagingtechnologies, prior to beginning the surgical operation.

The localizer is usually placed so that it has a field of view of thetrackers, that is, the localizer is positioned so that the target siteof the patient is within the target space of the localizer. The trackersinclude identifiable arrays of fiducials that are fixed to a surgicalinstrument or to a patient to move in concert with the surgicalinstrument or the patient, respectively. From the detected position ofthe trackers, the surgical navigation system can determine the positionand/or orientation of the surgical tool or patent anatomy, and monitorand track the position and/or orientation for changes over time. Theterm position refers to the three-dimensional coordinate values relativeto the surgical navigation system. The term orientation refers to thepitch, roll and yaw relative to the surgical navigation system or to areference orientation defined for the object. Collectively, the positionand the particular pitch, roll, and yaw values of a given orientationmay be referred to as the object's pose. When both the position andorientation (or pose) are defined, the object is known and trackable tothe surgical navigation system.

The tracker attached to the patient is often rigidly secured to the bonebeing treated, thereby maintaining a fixed relationship with respect tothe target site owing to the rigid nature of the bone, the rigidstructure of the tracker and the fixed securement therebetween. Inaddition to the trackers themselves being secured to the bone beingtreated, a further checkpoint screw is typically required. Using areference tracking array placed into contact with the checkpoint screw,and measuring the relative position of the checkpoint to the trackeraffixed to the bone, an operator can confirm the registration of thetracker to the bone and ensure that the mounting of the tracker to thebone remains true. Periodically performing this check during theoperation can increase the total time required for the operation. Byusing separate trackers on the surgical instrument and the patient, thetreatment end of the surgical instrument can be precisely positioned atthe target site by the surgeon aided by the navigation system.

During an initial phase of the operation, an object, whether a surgicaltool or a patient's anatomy, must be calibrated or registered to thesurgical navigation system. The process of calibration, or registration,refers to establishing a relationship between a physical object and itstracker to the virtual representations of the object and tracker as datawithin the surgical navigation system, that is, as virtual object dataand virtual tracker data, respectively. For example, preoperativeimaging of a patient's anatomy may be used to generate a 3D model ofthat anatomy as virtual object data in the memory and virtualenvironment of the surgical navigation system. Likewise, a surgical toolmay be manufactured according to known geometry and structure. Thisgeometry and structure may be represented in a 3D model of that tool asvirtual object data in the memory and virtual environment of thesurgical navigation system. To perform the calibration, additionalreference pointers or frames having additional tracker fiducial arraysmay be required to touch off reference points according to aregistration sequence.

Trackers, adapted for attachment to a patient's anatomy for use duringnavigation-guided surgery, typically include two or more bone pinsaffixed to a bone in close proximity. Placing multiple bone pins inclose proximity may require a single large incision much greater thanthe size of the bone pin itself. A clamp is secured to the bone pins,which also provides an interface for an array adapter. The array adaptermay provide rotational adjustments to allow the tracker to be preciselyarranged relative to the patient's anatomy and the localizer. The arrayadapter attaches to and supports the array frame. The frame is typicallyan expanded square, rectangular, diamond, or other geometric structure.The frame supports the reflective or emissive markers that focuses orprovides the signals in a wavelength suitable for the sensor technologyincluded in the localizer. The markers are typically arranged on theframe to maximize the distance between markers, such as at the cornersof a square or rectangular frame.

The accuracy of the navigation tracking is limited by the sensitivity ofthe imaging technology and the ability to relate the location of eachmarker relative to the other markers. Improving the quality of thetracking requires greater distance between the markers. On conventionaltrackers, this leads to ever larger frame components. Supporting largertracker frames may thus require larger bone pins or potential bonedamage as larger masses are cantilevered off bone tissue.

Therefore, there is a need in the art for improved trackers and methodsthat address the desire for improved accuracy while minimizing invasioninto tissue and bone during robotic surgery.

SUMMARY

This Summary introduces a selection of concepts in a simplified formthat are further described below in the Detailed Description. ThisSummary is not intended to limit the scope of the claimed subjectmatter, and does not necessarily identify each and every key oressential feature of the claimed subject matter.

In one example, a method for operating a navigation system is provided.A second tracker support is affixed to the rigid object and a secondplurality of trackable elements are secured to the second trackersupport. The first tracker support and the second tracker support areadapted to be independently secured to the rigid object and separated bya distance. A localizer is configured to track the first and secondplurality of trackable elements. The method comprises the navigationsystem defining a tracking arrangement to be tracked based on acombination of the first and second plurality of trackable elements. Thenavigation system registers a geometry of the tracking arrangementrelative to the rigid object and tracks the rigid object by detectingthe registered geometry of the tracking arrangement. The navigationsystem identifies a condition wherein at least one trackable element hasbeen displaced relative to the registered geometry and generates aresponse to address the identified condition.

In one example, a navigation system is provided. The navigation systemcomprises a first tracker support affixed to a rigid object and a firstplurality of trackable elements secured to the first tracker support. Asecond tracker support is affixed to the rigid object and a secondplurality of trackable elements are secured to the second trackersupport. The first tracker support and the second tracker support areadapted to be independently secured to the rigid object and separated bya distance. A localizer is configured to track the first and secondplurality of trackable elements. At least one controller is coupled tothe localizer and is configured to define a tracking arrangement to betracked based on a combination of the first and second plurality oftrackable elements. The at least one controller registers a geometry ofthe tracking arrangement relative to the rigid object. The rigid objectis tracked by detecting the registered geometry of the trackingarrangement. The at least one controller identifies a condition whereinat least one trackable element has been displaced relative to theregistered geometry, and generates a response to address the identifiedcondition.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the present disclosure will be readily appreciated as thesame becomes better understood with reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein like numbers denote like structures.

FIG. 1 is a perspective view of a surgical navigation system being usedin conjunction with a robotic surgical device.

FIG. 2 is a schematic view of a control system for controlling thesurgical navigation system and robotic surgical device.

FIG. 3 is a perspective view of coordinate systems used by a surgicalnavigation system in conjunction with an alternative robotic surgicalsystem.

FIG. 4A is a first form of a tracker affixed to a bone, shown at a firstangle.

FIG. 4B is a first form of a tracker affixed to a bone, shown at asecond angle.

FIG. 5A is a second form of a tracker in a first illustrated example.

FIG. 5B is a second form of a tracker in a second illustrated example.

FIG. 5C is a second form of a tracker in a third illustrated example

FIG. 6 is a third form of a tracker.

FIG. 7 shows trackers according to the second form affixed to a bone.

FIG. 8 shows trackers according to the third form affixed to a bone.

FIG. 9 is a flowchart illustrating the steps of a method for tracking.

FIGS. 10 and 11 illustrate visual guidance or instructions generated bythe navigation system to assist an operator in recovering registrationusing the trackers described herein.

DETAILED DESCRIPTION

Referring to FIG. 1, a surgical system 10 is illustrated for performingsurgery on a patient. The version shown in FIG. 1 includes a surgicalnavigation system 20. The surgical navigation system 20 is shown in asurgical setting such as an operating room of a medical facility. Thesurgical navigation system 20 is set up to track movement of variousobjects in the operating room. Such objects include, for example, asurgical instrument 22, a femur F of the patient, a tibia T of thepatient, and/or a robotic manipulator 56. The surgical navigation system20 tracks these objects for purposes of displaying their relativepositions and orientations to the surgeon and, in some cases, forpurposes of controlling or constraining movement of the surgicalinstrument 22 and/or robotic manipulator 56 relative to virtual cuttingboundaries associated, in the illustrated example, with the femur F andtibia T.

The surgical navigation system 20 includes a computer cart assembly 24that houses a navigation computer 26. A navigation interface is inoperative communication with the navigation computer 26. The navigationinterface includes a first display 28 adapted to be situated outside ofthe sterile field and a second display 29 adapted to be situated insidethe sterile field. The displays 28, 29 are adjustably mounted to thecomputer cart assembly 24. First and second input devices (not shown)such as a keyboard and mouse can be used to input information into thenavigation computer 26 or otherwise select/control certain aspects ofthe navigation computer 26. Other input devices are contemplatedincluding a touch screen 30, gesture control, or voice-activation.

A localizer 34 communicates with the navigation computer 26. In theexample shown, the localizer 34 is an optical localizer and includes acamera unit 36. The camera unit 36 has an outer casing 38 that housesone or more optical sensors 40. In some examples at least two opticalsensors 40 are employed. The optical sensors 40 are capable of variableattenuation of radiant energy, for example, light, into signals as smallbursts of electrical current that convey information. The camera unit 36may also include a video camera 41 or other additional sensing device.

The optical sensors 40 may be separate charge-coupled devices (CCD). Insome examples, two, two-dimensional CCDs are employed. In some cases,the optical sensors 40 are arranged for stereoscopic operation, orsingle cameras combined with depth sensors, laser range finders, and thelike, may be used. It should be appreciated that in other examples,separate camera units, each with a separate CCD, or two or more CCDs,could also be arranged around the operating room. The optical sensors 40may include CCDs capable of detecting infrared (IR) radiant energy. Inalternative examples, the optical sensors may employ other sensingtechnology, including, but not limited to, complimentary metal-oxidesemiconductor (CMOS) active-pixel sensors, and the like.

The camera unit 36 may be mounted on an adjustable arm or otherarticulated support structure of the cart assembly 24 to selectivelyposition the localizer 34 with a, preferably unobstructed, field of viewof the target space including the surgical setting within which will bethe patient anatomy and trackers, as discussed below. In some examples,the camera unit 36 is adjustable in at least one degree of freedom byrotating about a rotational joint. In other examples, the camera unit 36is adjustable about two or more degrees of freedom.

The camera unit 36 includes a camera controller 42 in communication withthe optical sensors 40 to receive signals from the optical sensors 40.The camera controller 42 communicates with the navigation computer 26through either a wired or wireless connection (not shown). One suchconnection may be an IEEE 1394 interface, which is a serial businterface standard for high-speed communications and isochronousreal-time data transfer. The connection could also use a companyspecific protocol. In other examples, the optical sensors 40 maycommunicate directly with the navigation computer 26, such that thenavigation computer incorporates the functionality of, and thus operatesas, the camera controller 42. Processing of the signals from the opticalsensors 40 may occur at the camera controller 42. Alternatively, thecamera controller 42 may communicate the signals to the navigationcomputer 26 for processing.

The navigation computer 26 can be a personal computer or laptopcomputer. The navigation computer 26 has the display 28, centralprocessing unit (CPU) and/or other processors, memory (not shown), andstorage (not shown). The navigation computer 26 is loaded with softwareas described below. The software is configured to execute algorithms forperforming any of the functionality and steps described herein. Thesoftware converts the signals received from the camera unit 36 or theoptical sensors 40 into data representative of the position andorientation of the objects being tracked. Position and orientationsignals and/or data is used by the navigation computer 26 for purposesof tracking objects. The computer cart assembly 24, display 28, andcamera unit 36 may be like those described in U.S. Pat. No. 7,725,162 toMalackowski, et al. issued on May 25, 2010, entitled “Surgery System,”hereby incorporated by reference.

The surgical system 10 illustrated in FIG. 1 includes a plurality oftracking devices 44, 46, 48, also referred to herein simply as trackers.In the illustrated example, two trackers 44, 46 are coupled to the tibiaT of the patient.

An instrument tracker 48 is coupled to the surgical instrument 22. Theinstrument tracker 48 may be integrated into the surgical instrument 22during manufacture or may be separately mounted to the surgicalinstrument 22 in preparation for the surgical procedure. The working endof the surgical instrument 22, which is being tracked by virtue of theinstrument tracker 48, may be an energy applicator EA such as a rotatingbur, saw blade, electrical ablation device, or the like. The energyapplicator EA may be a separate component such as a replaceable bur, sawblade, ablator, or the like that is releasably connected to a handpieceof the surgical tool 22 or the energy application EA may be integrallyformed with the handpiece.

The trackers 44, 46, 48 may be active trackers or passive trackers. Ingeneral, any passive or active tracker or marker can be considered atrackable element. The active or passive trackers can have variousconfigurations. The active tracker has its own power source and may bebattery powered with an internal battery or may have leads to receivepower through the navigation computer 26, which, like the camera unit36, may receive external power. The active tracker may have an array offiducials (also referred to as tracking elements or markers) thatactively generate and emit radiation in a wavelength detectable by theoptical sensors 40. The fiducials of an active tracker may be a lightemitting diode (LED), including, for example, an infrared LED. The arrayof active fiducials may be “always on” or may be operative toselectively fire, that is emit radiation, according to and in responseto commands from the surgical navigation system 20. In suchselective-fire active trackers, the tracker may communicate by way of awired or a wireless connection with the navigation computer 26 ofsurgical navigation system 20. Other examples of active trackers arecontemplated, such as other type of optical elements besides LEDs,electromagnetic trackers, radio frequency trackers, or any other type oftracker whereby a power source on the tracker can energize an elementthat can be detected by the navigation system (using optical sensing orany other modality).

Additionally or alternatively, the tracker may include passive trackersor markers. Unlike the active tracker, the passive tracker array doesnot require a power source. In one example, passive trackers focus orreflect ambient radiation or radiation that has been emitted into thetarget space, for example by one or more infrared LEDs provided on thecamera unit 36 or elsewhere associated with the surgical system 10. Thepassive trackers or markers may have various configurations. The passivetrackers may be optical or non-optical. For example, non-optical passivetrackers may include passive electromagnetic trackers, passive radiofrequency trackers, or any other type of passive element that canreflect or radiate a signal back to the navigation system (using anymodality). In other examples, the passive markers can be unique shapes,patterns, barcodes, etc., that can be optically identified by thenavigation system, e.g., using machine vision. Any of the techniques,trackers, or system examples described herein may utilize anycombination and configuration of active and passive trackers

In the example shown, the surgical instrument 22 is attached to asurgical manipulator 56. Such an arrangement is shown in U.S. Pat. No.9,119,655 to Bowling et al, issued Sep. 1, 2015, entitled, “SurgicalManipulator Capable of Controlling a Surgical Instrument in MultipleModes,” the disclosure of which is hereby incorporated by reference.

In other examples, the surgical instrument 22 may be manually positionedby only the hand of the user, without the aid of any cutting guide, jig,or other constraining mechanism such as a manipulator or robot. Such asurgical instrument is described in U.S. Pat. No. 9,707,043 to Bozung etal., issued Jul. 18, 2017, the disclosure of which is herebyincorporated by reference.

Initially, the objects to be located are viewed by the optical sensors40 and identified. The objects may be identified by selecting theobjects to be tracked using an input device connected to the machinevision controller 14 or navigation computer 26. The navigation computer26 may store detailed information regarding numerous objects in memoryor data storage on the navigation computer 26 and the user may be ableto manually select the objects to be tracked from a database of objects.Although described generally herein with regard to trackers attached toa patient's anatomy, for example, a bone, the present disclosurecontemplates that objects beyond anatomical objects may be tracked.Surgical tools, reference frames, robotic manipulators or other objectsmay be tracked using the systems and methods as disclosed herein.

Using the disclosed systems and methods, an operator may desire todefine a volume to be tracked that is not limited to the physicalboundaries of a particular item. The operator may desire to define anadditional margin of space around an item to be avoided, for example,where a robotic manipulator 56 is in use. The navigation computer 26 maypresent a visual interface to allow the operator to designate whichmarkers define the tracker and allow the operator to designate theobject, including in some cases a specified volume in space, to betracked.

Additionally, or alternatively, the navigation computer 26 may identifythe objects to be tracked based on a pre-operative surgical plan. Inthis case, the navigation computer 26 may have a preset list of workflowobjects that may be used in the pre-scripted surgical workflow. Thenavigation computer 26 may actively search for and locate the workflowobjects using software. For instance, groups of pixels associated withdifferent sizes and shapes of the various objects may be stored in thenavigation computer 26. By selecting/identifying the objects to belocated/tracked, the software identifies the corresponding group ofpixels and the software then operates to detect like groups of pixelsusing conventional pattern recognition technology.

Additionally, or alternatively, the objects to be located/tracked can beidentified using an interface in which one of the operators outlines orselects the objects to be tracked on one or more of the displays 28, 29.For instance, images taken by the optical sensors 40, or video camera41, of the surgical site may be displayed on one or more of the displays28, 29 (and/or other displays). The operator then, using a mouse,digital pen, or the like, traces objects to be located/tracked on thedisplay 28 and/or 29. The software stores the pixels associated with theobject that was traced into its memory. The operator (or other user) mayidentify each object by a unique identifier such as naming the objectusing the software so that the saved group of pixels may be associatedwith the unique identifier. Multiple objects could be stored in thismanner.

The navigation computer 26 utilizes conventional pattern recognition andassociated software to later detect these objects. The navigation system20 is able to detect movement of these objects by continuously takingimages, reviewing the images, and detecting movement of the groups ofpixels associated with the objects.

Further, in addition or in the alternative, the object to belocated/tracked can be identified through recognition of the object, orthe tracker or trackers attached to the object. The particularconfiguration of the markers on the tracker can be sufficient toidentify to the navigation computer 26 the object of interest when thatconfiguration is compared against stored tracker information. In aparticular example, the linear distance between markers may bemeasurable by the navigation computer 26 and sufficient to identifytracker to the navigation computer 26. In this example, two trackerseach having two markers can differentiate between two objects based ontwo different separation distances between the markers on the respectivetrackers. Other characteristic configurations that may enable thisidentification include the size of markers, shape, color or otherpatterns.

In conventional surgical navigation systems, the objects to be trackedare initially located and registered using a navigation pointer P. Forexample, the navigation pointer P may have an integrated tracker PT. Thenavigation computer 26 may store initial data corresponding to alocation of the tip of the pointer P relative to the tracker PT suchthat the navigation system 20 is able to locate and track the tip of thepointer P in the localizer coordinate system LCLZ. Accordingly, prior tothe start of the surgical procedure, once all the objects are located intheir desired locations, one of the participants may touch all of theobjects with the pointer P, while identifying the objects in thenavigation system 20 using one of the input devices described above. So,for example, when the participant touches the surgical instrument 22with the tip of the pointer P, the participant may simultaneouslytrigger collection of that point in the localizer coordinate system LCLZ(via another input device, such as a foot pedal). When the point iscollected, the participant can also enter into the navigation softwarethe identity of the object (via typing, pull-down selection from a listof objects, etc.).

Referring to FIG. 2, a schematic view of a control system forcontrolling the surgical navigation system 20 and robotic surgicaldevice 56 is shown. In some examples the trackers 44, 46 includefiducials that are connected to a tracker controller (not shown) locatedin a housing (not shown) of the associated tracker 44, 46 thattransmits/receives data to/from the navigation computer 26. In oneexample, the tracker controllers transmit data through wired connectionswith the navigation computer 26. In other examples, a wirelessconnection may be used. In these examples, the navigation computer 26has a transceiver (not shown) to receive the data from the trackercontroller. In other examples, the trackers 44, 46 may have passivemarkers 50, such as reflectors that reflect light, for example, lightemitted by an LED provided on camera unit 36 light. For example, thecamera unit 36 may include complementary emitters in a wavelength towhich the optical sensors 40 are sensitive. The reflected light is thenreceived by the optical sensors 40. In some examples, the trackers 44,46 may also include a gyroscope sensor 60 and accelerometer 70, such asthe trackers shown in U.S. Pat. No. 9,008,757 to Wu, et al., issued onApr. 14, 2015, entitled, “Navigation System Including Optical andNon-Optical Sensors,” the entire disclosure of which is herebyincorporated by reference. These additional sensors 60, 70, may provideinformation to the navigation computer 26 for use by the navigationcomputer to determine or track the trackers' 44, 46 position ororientation.

The navigation computer 26 includes a navigation processor 52. It shouldbe understood that the navigation processor 52 could include one or moreprocessors to control operation of the navigation computer 26. Theprocessors can be any type of microprocessor or multi-processor system.The term “processor” is not intended to limit the scope of the inventionto a single processor.

As illustrated in FIG. 2, the camera unit 36 receives optical signals 53from the fiducials 50 of the trackers 44, 46 and outputs to theprocessor 52 signals relating to the position of the fiducials 50 of thetrackers 44, 46 relative to the localizer 34. Based on the receivedoptical (or non-optical signals, in some examples), navigation processor52 generates data indicating the relative positions and orientations ofthe trackers 44, 46, 48 relative to the localizer 34.

Prior to the start of the surgical procedure, additional data are loadedinto the navigation processor 52. Based on the position and orientationof the trackers 44, 46, 48 and the previously loaded data, such asvirtual object data representing the geometry of the object to which thetracker is attached, navigation processor 52 determines the position ofthe working end of the surgical instrument 22 (e.g., the centroid of asurgical bur) and the orientation of the surgical instrument 22 relativeto the tissue against which the working end is to be applied. In someexamples, navigation processor 52 forwards this data to a manipulatorcontroller 54. The manipulator controller 54 can then use the data tocontrol a robotic manipulator 56 as described in U.S. Pat. No. 9,119,655to Bowling, et al., incorporated above.

The navigation processor 52 also generates image signals that indicatethe relative position of the surgical instrument working end to thetissue. These image signals are applied to the displays 28, 29. Displays28, 29, based on these signals, generate images that allow the surgeonand staff to view the relative position of the surgical instrumentworking end to the surgical site. The displays, 28, 29, as discussedabove, may include a touch screen 30 or other input/output device thatallows entry of commands.

In the example shown in FIG. 1, the surgical tool 22 forms part of anend effector of the manipulator 56. The manipulator 56 has a base 57, aplurality of links 58 extending from the base 57, and a plurality ofactive joints (not numbered) for moving the surgical tool 22 withrespect to the base 57. The links 58 may form a serial arm structure asshown in FIG. 1, a parallel arm structure (shown for example in FIG. 3),or other suitable structure. The manipulator 56 has the ability tooperate in a manual mode in which a user grasps the end effector of themanipulator 56 in order to cause movement of the surgical tool 22 (e.g.,directly, through force/torque sensor measurements that cause activedriving of the manipulator 56, or otherwise) or a semi-autonomous modein which the surgical tool 22 is moved by the manipulator 56 along apredefined tool path (e.g., the active joints of the manipulator 56 areoperated to move the surgical tool 22 without requiring force/torque onthe end effector from the user). An example of operation in asemi-autonomous mode is described in U.S. Pat. No. 9,119,655 to Bowling,et al., incorporated above. A separate tracker (not shown) may beattached to the base 57 of the manipulator 56 to track movement of thebase 57.

The manipulator controller 54 may have a central processing unit (CPU)and/or other manipulator processors, memory (not shown), and storage(not shown). The manipulator controller 54, also referred to as amanipulator computer, is loaded with software as necessary to controland operate the manipulator. The manipulator processors could includeone or more processors to control operation of the manipulator 56. Themanipulator 56 may be in the form of a conventional robotic system orother conventional machining apparatus, and thus the components thereofshall not be described in detail.

The manipulator controller 54 determines the desired location to whichthe surgical tool 22 should be moved. Based on this determination, andinformation relating to the current location (e.g., pose) of thesurgical tool 22, the manipulator controller 54 determines the extent towhich each of the plurality of links 58 needs to be moved in order toreposition the surgical tool 22 from the current location to the desiredlocation. The data regarding where the plurality of links 58 are to bepositioned is forwarded to joint motor controllers (not shown) (e.g.,one for controlling each motor) that control the active joints of themanipulator 56 to move the plurality of links 58 and thereby move thesurgical tool 22 from the current location to the desired location.

Referring to FIG. 3, tracking of objects is generally conducted withreference to a localizer coordinate system LCLZ. The localizercoordinate system has an origin and an orientation (a set of x-, y-, andz-axes). During the procedure, one goal is to keep the localizercoordinate system LCLZ in a known position. An accelerometer (not shown)mounted to the camera unit 36 may be used to track sudden or unexpectedmovement of the localizer coordinate system LCLZ, as may occur when thecamera unit 36 is inadvertently bumped by surgical personnel.

Each tracker and object being tracked also has its own coordinate systemseparate from localizer coordinate system LCLZ. Components of thenavigation system 20 that have their own coordinate systems are the bonetrackers 44 and 46, and the instrument tracker 48. These coordinatesystems are represented as, respectively, bone tracker coordinatesystems BTRK1 and BTRK2, and instrument tracker coordinate system TLTR.

Navigation system 20, through the localizer 34 monitors the positions ofthe tibia T of the patient by monitoring the position of bone trackers44, 46 coupled to the bone. The tibia coordinate system is TBONE, whichare the coordinate systems of the bone to which the trackers 44, 46 arecoupled.

Prior to the start of the procedure, pre-operative images of the tibia Tare generated (or of other tissues in other examples). These images maybe based on MRI scans, radiological scans or computed tomography (CT)scans of the patient's anatomy. These images are mapped to the tibiacoordinate system TBONE using well known methods in the art. Theseimages are fixed in the tibia coordinate system TBONE. As an alternativeto taking pre-operative images, plans for treatment can be developed inthe operating room (OR) from kinematic studies, bone tracing, and othermethods. In one example, the other methods may include morphed genericor statistical models.

During an initial phase of the procedure, the bone trackers 44, 46 arecoupled to the bones of the patient. The pose (position and orientation)of coordinate system TBONE must be mapped to coordinate systems BTRK1and BTRK2, respectively. Given the fixed relationship between the bonesand their bone trackers 44, 46, positions and orientations of the tibiaT in the tibia coordinate system TBONE must be transformed to the bonetracker coordinate systems BTRK1 and BTRK2 so the camera unit 36 is ableto track the tibia T by tracking the bone trackers 44, 46. Thispose-describing data are stored in memory integral with both manipulatorcontroller 54 and navigation processor 52.

The working end of the surgical instrument 22 (also referred to asenergy applicator distal end) has its own coordinate system EAPP. Theorigin of the coordinate system EAPP may represent a centroid of asurgical cutting bur, for example. The pose of coordinate system EAPPmust be fixed to the pose of instrument tracker coordinate system TLTRbefore the procedure begins. Accordingly, the poses of these coordinatesystems EAPP, TLTR relative to each other must be determined in thenavigation computer 26. The pose-describing data are stored in memoryintegral with both manipulator controller 54 and navigation processor52.

Referring back to FIG. 2, a localization engine 62 is a software modulethat may be included within the navigation system 20. Components of thelocalization engine 62 may execute on navigation processor 52. In someexamples, however, the localization engine 62 may execute on themanipulator controller 54.

Localization engine 62 receives as inputs the optically-based signalsfrom the camera controller 42 and, in some examples, the non-opticallybased signals from the tracker controller. Based on these signals,localization engine 62 determines the pose of the bone trackercoordinate systems BTRK1 and BTRK2 in the localizer coordinate systemLCLZ. Based on the same signals received for the instrument tracker 48,the localization engine 62 determines the pose of the instrument trackercoordinate system TLTR in the localizer coordinate system LCLZ.

The localization engine 62 forwards the signals representative of theposes of trackers 44, 46, 48 to a coordinate transformer 64. Coordinatetransformer 64 is a navigation system software module that runs onnavigation processor 52. Coordinate transformer 64 references the datathat defines the relationship between the pre-operative images of thepatient and the bone trackers 44, 46. Coordinate transformer 64 alsostores the data indicating the pose of the working end of the surgicalinstrument relative to the instrument tracker 48.

During the procedure, the coordinate transformer 64 receives the dataindicating the relative poses of the trackers 44, 46, 48 to thelocalizer 34. Based on these data and the previously loaded data, thecoordinate transformer 64 generates data indicating the relativeposition and orientation of the coordinate system EAPP, and the bonecoordinate systems, FBONE and TBONE to the localizer coordinate systemLCLZ.

As a result, coordinate transformer 64 generates data indicating theposition and orientation of the working end of the surgical instrument22 relative to the tissue (e.g., bone) against which the instrumentworking end is applied. Image signals representative of these data areforwarded to displays 28, 29 enabling the surgeon and staff to view thisinformation. In certain examples, other signals representative of thesedata can be forwarded to the manipulator controller 54 to guide themanipulator 56 and corresponding movement of the surgical instrument 22.

In a similar manner, other trackers may be coupled to any other suitableobject to be tracked within the operating room, and each object andassociated tracker may be registered to the localizer coordinate systemLCLZ as described above.

An improved tracker array, and improved method of registration, orcalibration, provide for improved tracking accuracy and stability and isless invasive to tissue and bone. In accordance with the presentdisclosure, first and second bone pins are affixed to a patient'sanatomy, a plurality of fiducial markers are secured to the first andsecond bone pins or a combination of the first and second bone pins, acustom tracking arrangement of fiducials is defined within thenavigation computer 26 incorporating the plurality of fiducial markerssecured to the first and second bone pins and stored as a trackingarray. The tracking array may have its own tracker coordinate system, orit may be a combination of the coordinate systems of fiducial markerssupported by bone pins, plates or screws.

FIGS. 1-3 depict an improved tracker array comprising the trackers 44and 46. Each of tracker 44 and 46 includes a tracker support which canbe affixed to the object. Each of tracker 44 and 46 includes one or morefiducials or markers mounted on the tracker support. As described above,and in alternative examples below, the tracker support may comprise abone pin, screw, plate or other suitable supporting structure mountableto an object to be tracked. In the tracker 44 and 46 example illustratedin FIGS. 1-3, the tracker support comprises a single bone pin affixed tothe patient's tibia which extends integrally into a planar surface atthe distal end. As described above, and in alternative examples below,the fiducials or markers may be include active LED markers, or passivereflective markers. In the tracker 44 and 46 example illustrated inFIGS. 1-3, the markers comprise adhesive reflectors 50 affixed to thetracker supports. In alternative examples, the markers may bepre-positioned on the tracker support, for example as laser-etchedmarkings, inked markings, or adhered markings.

In a registration workflow for registering a patient's tibia, thetrackers 44, 46 may include tracker supports that are pre-marked with anarrangement recognizable by the navigation computer, either throughdimensional distribution or placement of the markers, color, size, shapeor other designating characteristic. The navigation computer, storinginformation correlating to the patient's anatomy and to the patterns ofmarkers on the trackers 44, 46, may automatically register the trackers44, 46 to the anatomy (tibia T) for tracking during the procedure.

In alternative examples, where the trackers 44, 46 are not pre-markedand the navigation computer 26 not being provided pre-operative imagingof the patient's anatomy, during a registration workflow, the operatormay be instructed to register, for example, the patient's tibia. In oneexample workflow, the operator would be instructed to attach a first andsecond tracker support to the object to be tracked, in this case—thepatient's tibia T. Not being pre-marked, the operator would then beinstructed to secure a plurality of markers 50 to the first and secondtracker supports. The navigation computer 26, displaying images of thesurgical space, may prompt the operator to define the marker arrangementforming the tracker by designating the markers securing to the trackersupports shown within the displayed images.

At this step, the navigation computer may evaluate the defined markerarrangement to ensure that the tracker is sufficiently unique to bedifferentiated from other trackers that may be registered within thenavigation computer 26. If potential conflict is determined, thenavigation computer 26 may prompt the user to further differentiate thetracker, by doing one or more of adding or removing markers 50 from thetracker supports 44, 46, modifying the placement of one or more markerson the tracker supports 44, 46, or adding an additional tracker supportwith yet more markers.

Continuing with the example workflow described, once the navigationcomputer accepts the defined marker arrangement forming the tracker, thetracker is registered to the object, in this example, the patient'stibia, T. Where the navigation computer 26 is not provided withpre-operative models or other geometry corresponding with the object tobe tracked, the operator may provide input to designate the object basedon imaging, perioperative scans, or other intraoperative tools. In oneexample, the navigation computer may prompt the operator to select theobject to be tracked based on stereoscopic imaging of the patient'sanatomy. The navigation computer 26 may further utilize morphed atlasmodels of the relevant anatomy to register the defined markerarrangement to the patient's anatomy. Once registered, the navigationcomputer may track the object based on the detected movement of themarker arrangements.

During tracking, the navigation computer 26 may be configured to monitorthe tracking quality to ensure accurate and precise tracking ismaintained. In one example, the navigation computer 26 may be configuredto monitor and detect whether any marker in the defined markerarrangement deviates from the expected position relative to the othermarkers in the arrangement. If such a change is detected, it mayindicate that one of the tracker supports may have come loose from theanatomy or be bent or otherwise displaced during the operation. Thenavigation computer 26 may generate a visible, or audible alarm to alertthe operator that the tracking may have been compromised and that theobject may need to be re-registered in the system. Upon detecting anerror with the defined marker arrangement, the navigation computer 26may be further configured to communicate an interrupt to the manipulatorcontroller 54 to cause the robotic manipulator to stop its motion or toretreat to a safe position at a distance from the patient's anatomy,other tools, or operators in the surgical environment.

An improved tracker 100 is shown in another example in FIGS. 4A and 4Bto address and eliminate shortcomings associated with conventionalsurgical trackers. Tracker 100 is shown from two angles affixed to bone101. Although illustrated as a femur, bone 101 could be any suitablebone or other tissue. The tracker 100 is affixed with two bone pins 102disposed at opposite ends of bone 101. The bone pins 102 are adapted tosupport the tracker 100 in a rigid relationship with the patient'sanatomy.

The bone pin 102 includes a feature 104 to allow the bone pin 102 tointerface with a tool for inserting bone pin 102 as a drive feature. Oneexample of feature 104 includes a plurality of flat surfaces of apredefined dimension to allow a tool to grasp the bone pin 102. Anotherexample of feature 104 may include a spline, knurling, hex socket orother geometry configured to aid the transmission of torsional forcefrom the tool to the bone pin 102. In some alternatives, the bone pin102 excludes a drive feature, for example as a smooth cylinder. In suchcases, a driver may clamp onto an accessible surface of the pin.

The bone pin 102 includes an elongated shaft 106 extending from thefeature 104. Disposed along the length of the shaft 106 is a mountinginterface 108. The mounting interface 108 may include a section of theshaft 106 that is threaded, conical or stepped, that is, a length of theshaft 106 may be of non-constant diameter. The mounting interface 108provides a support point to secure plate 110 to the bone pins 102. Themounting interface may be structured so as to allow the plate 110 to besecured at a variable height relative to the bone pin 102. For example,where mounting interface 108 includes a threaded length of shaft 106,the height of the plate may be secured along the length of shaft 106 byrotating a nut (not shown) with complimentary threads. Similarly,separate nuts could be threaded on the shaft 106 on opposed sides of theplate 110 and tightened toward each other to secure the plate inposition on the shaft 106. In further alternatives, tape, glue, curableor self-curing bonding materials can be used to secure the plate.

The plate 110 is supported at its respective ends by the bone pins 102affixed in bone 101. In the form shown, plate 110 includes multiplesurface portions 112, 114, 116 angled with respect to each other. Thesurface portions 112, 114, 116, may advantageously be positioned so thatthe visibility of the surfaces are maintained to the localizer even asthe patient's anatomy is moved through multiple positions. In onealternative example, the plate can be formed or re-formed during theprocedure. The plate may be provided with bendable portions that can beselectively adjusted to optimize the visibility of plate surfaces to thelocalizer. The placement and adjustment would occur prior to theregistration of the object and tracker within the navigation computer26. Once tracking was initiated, following registration, the staticconfiguration of the tracker components relative to other componentsmaking up the tracker, and the static relationship between the trackerand the object, is generally used by the navigation computer to ensurethat accurate and precise tracking is maintained. The navigationcomputer 26 may be configured to periodically check and confirm therelative relationship between individual markers comprising the tracker,or a portion or the tracker relative to other portions, to ensure thattracking has not been impeded. Upon detecting a change in the trackerconfiguration, either internally or relative to the tracked object, thenavigation computer 26 may generate a warning alarm, such as an audibleor visible alert. Additionally, the navigation computer 26 may interruptthe manipulator controller 54, directing it to cease operation of themanipulator arm 58 or withdraw the instrument 22 to a safe position.

Affixed to one or more of the surface portions 112, 114, 116 of plate110 are markers 118. The markers 118 are the fiducial points of thetracker that allow the localizer to locate and track the object, in thiscase bone 101, to which the tracker 100 is affixed. The markers 118 maybe one or more of adhesive-backed markers, laser-etched marks, inked orprinted marks, or other suitable marking detectable by the localizer 34.Although depicted as circles, alternative markers 118 may include othershapes and figures. In particular, the markers 118 may be particularlyconfigured to provide human-readable or machine-readable information. Inone example, an “F” form may be provided to designate a tracker attachedto a patient's Femur, and a “T” from may designate attachment to apatient's Tibia. Similarly, marker 118 may include one or more bar code,QR code, color pattern or other information encoding markingrecognizable to the navigation computer 26.

In one example, marking 118 is an adhesive-backed IR-reflective sticker.In this example, the localizer 34 includes IR-sensitive optical sensors40 and one or more IR-emitting LEDs provided on the camera unit 36. Atthe beginning of a surgical operation, two bone pins 102, as first andsecond tracker supports, are affixed to bone 101. In this example, thebone pins 102 are inserted into the bone using a powered drill having aJacobs chuck to rotate the bone pins 102 so that they screw into thebone 101. The bone pins 102 include a self-drilling end feature 120 andare threaded along their length to secure to the bone 101.

In the illustrated example, the bone pins 102 are shown at opposite endsof a femur bone 101. In other applications, bone 101 may be any othersuitable bone. For example, bone 101 may be a tibia. From patient topatient, the length of the bone will vary and therefore the distancebetween the bone pins 102 will likewise vary. Therefore, the plate 110must accommodate different separation distances.

The plate 110 may be provided with multiple apertures 121 so thatappropriate separation distance between the bone pins 102 may beselected for the bone to be tracked. The apertures 121 may be formed inthe plate 121, for example, by drilling through the plate. An operatormay use at least one aperture 121 formed in the plate 110 prior tobeginning an operation, and may form a second aperture during theprocedure by selecting an appropriate location on the plate 110 inrelation to the patient's anatomy for a second secure point of mounting.To accommodate a wider range of distances between bone pins 102, plate110 may include a circular aperture 121 at one end and include anaperture formed as an elongated slot (indicated in the FIGS. 4A, 4B indashed lines). Multiple plate 110 options may be available havingdifferent lengths so that a suitable plate 110 can be selected for theparticular application.

The plate 110 mounts to the bone pins 102 to be supported thereon at aheight above the skin of the patient. The height at which the plate 110mounts to the bone pins 102 may, in some cases, be directly adjacent andin contact with the patient's skin. The incision needed for the tracker100 may be less invasive than needed for conventional trackers.Conventional trackers may require multiple bone pins to be placed inclose proximity so that a single, large incision is needed to encompassboth pins. The tracker 100, having bone pins 102 disposed at oppositeends of the bone allows separate, smaller incisions that may provide thepatient faster recovery.

The plate 110 may be pre-marked, or markers 118 may be placed on theplate 110 after it has been mounted to the bone pins 102. Inapplications where the plate is pre-marked, for example withlaser-etched markings, the placement of the markers 118 on the plate 110may be in a pre-defined arrangement. The pre-defined arrangement ofmarkers 118 may correspond to one of a known library of pre-definedarrangements corresponding to identifiable objects. For example, in oneapplication the particular arrangement of laser-etched markers may bestored in the navigation computer 26 to correspond to and identify theobject as a femur. In such an application, alternative plates may bepre-marked with marker arrangements to identify one or more bones to betracked as part of the surgical operation. The operator would select theappropriate plate 110 to mount to the proper bone so that the navigationcomputer 26 associates the tracker 100 with the object based on therecognized pattern of markers 118 on the tracker 100.

In applications where the plate 110 is not pre-marked, the operatorwould apply markers 118, such as adhesive-backed stickers, to create anarrangement to be identified to the navigation computer 26 and whichthereafter can be tracked by the navigation system 20. Once applied toplate 110, the operator may identify to the navigation system 20 themarkers comprising the tracker 100 by providing input and/or makingselections through touch screen 30. For example, in an operation wheremultiple bones are being tracked, the operator may need to differentiatethe markers 118 applied to a plate 110 affixed to a femur from a plateaffixed to a tibia. The tracker 100 in this example allows the in situdefinition of a marker arrangement to track and identify a particularobject.

An alternative form of a tracker 122 according to the present disclosureis illustrated in FIG. 5A. The tracker 122 includes a bone pin 102 andsleeve 124. The bone pin 102 is affixed to the bone or object to betracked as described above. After the bone pin 102 is affixed, sleeve124 is mounted to the bone pin 102. The sleeve 124, similar to plate110, is supported on the bone pin 102 at a height along the bone pin 102a distance from the patient's anatomy. The sleeve 124 includes acomplementary feature (not shown) to interact with mounting interface108 and allow the sleeve 124 to be secured to the bone pin 102, and mayinclude, for example, a set screw.

Supported on the sleeve 124 are one or more fiducial markers 128. In oneexample, the markers 128 are reflector balls provided with a material orcoating adapted to reflect infrared light. In another example, themarkers 128 are LEDs that emit light in the near infrared wavelength. Inthis way, the markers 128 provide signals 53 to the optical sensors 40of the localizer 34. The sleeve 124 and markers 128 may have a knowngeometry that can be stored in the navigation computer 26. Specifically,the markers 128 may be spaced apart along the sleeve 124 by a defineddistance. Additionally, the markers 128 may be a defined radial distancefrom a center point or axis of the bone pin 102. The sleeve 124 may berotatable about the bone pin 102 to be positioned on the bone pin 102for optimal visibility to the localizer 34. The sleeve 124 may furtherbe securable to the bone pin 102 once positioned so that furtherrotation is restricted. For example, one or more set screws may be usedto secure the sleeve at a desired position and/or orientation along thebone pin 102. In alternative examples, tape, glue, curable orself-curing bonding materials can be used to secure the sleeve 124 tothe bone pin 102. In the example shown, the sleeve 124 is generallycylindrical and capable of rotation about the bone pin 102 and slidingrelative to the bone pin 102 prior to being secured in place to maintaindirect visual line-of-sight to the localizer 34.

A further alternative form of a tracker 123 is illustrated in FIG. 5B.The tracker 125 includes a similar bone pin 102 as in the prior example.In place of the sleeve 124 controlling a fixed distance between themarkers 128, the markers 128 in FIG. 5B are separately supported onrings 129. The rings 129 may include complementary features to interactwith a mounting interface 108 on the bone pin 102 to allow the rings 129to be secured to the bone pin 102. In alternative examples, tape, glue,curable or self-curing bonding materials can be used to secure the rings129 to the bone pin 102.

Similarly as with the sleeve 124 described above, the rings 129 can berotated and slid relative to the bone pin 102 prior to being secured inplace. Each ring 129 allows the respective marker 128 to be selectivelypositioned on the bone pin. This allows the multiple markers 128 alongthe bone pin 102 to be at different rotational positions. That is, themarkers 128 may not be aligned along one side of the bone 102, unlikethe configuration illustrated in FIG. 5B. In a further alternative, thering 129 may be configured to support the marker 128 at differentdistances away from the bone pin 102. Once the bone pin is inserted, themarkers 128 may be positioned in the best orientation to be visible tothe localizer 34.

Another alternative tracker 125 is illustrated in FIG. 5C. The tracker125 likewise includes a bone pin 102. In this example, the marker 127 isa spherical marker that can slide over the bone pins 102. The sphericalmarkers 127 offer improved visibility over the markers 128 mountedoffset from the bone pin 102. The markers 127 are visible from any sideof the bone pin 102, and is not blocked by the bone pin 102 in the viewof the localizer 34.

A related alternative form of a tracker 130 according to the presentdisclosure is illustrated in FIG. 6. The tracker 130 likewise includesbone pin 102, but the fiducial support 138 is offset from the bone pin102 in this form. Rather, supported on bone pin 102, collar 132 isdisposed along the length of the shaft 106 and is secured to themounting interface 108. The collar 132 further supports a post 136extending from the collar 132. A clamp 140 secures to the post 136 andallows the support 138 to be adjusted and positioned prior to beingrigidly affixed in place. Extending from the clamp 140, stem 142 allowsthe fiducial markers 146 to be further offset from the bone pin 102. Theclamp 140 and post 136 may be like those shown in U.S. PatentApplication Publication No. 2017/0340395 to Perez, entitled, “NavigationTracker With Kinematic Connector Assembly,” hereby incorporated hereinby reference. In alternative examples, markers 146 may be arrangedsimilarly as shown in FIGS. 5A-5C. For example, stem 142 may extend toprovide sufficient mounting space for markers 146 to be independentlymounted on rings (not shown, but see FIG. 5B); or as spherical markers(see FIG. 5C).

Stem 142 may be formed of a bendable and/or malleable material thatallows the operator to impart a plastic deformation to the stem prior tothe operation to precisely arrange the markers in the desired position.The stem 142 may be formed of a strain hardening material that resistssubsequent deformation after an initial adjustment is made. Examplematerials include titanium, its alloys, copper, brass, and spring steel.The markers 146 are supported on shaft 144, which rigidly retains themarkers 146 in a static positional relationship relative to each other.

Similar to the sleeve 124 and markers 128, the shaft 144 supports themarkers 146 in a known geometry that can be stored in the navigationcomputer 26. Specifically, the markers 146 may be spaced apart along theshaft 144 by a defined distance. The markers 146 may be a defined radialdistance from a center point or axis of the shaft 144. The shaft 144 maybe rotatable about the stem 142 to be positioned for optimal visibilityto the localizer 34. This rotation could be provided by virtue of themalleability of the stem 142, if the shaft 144 and stem 142 are fixedtogether, or the shaft 144 may be a separate component from the stem142. The shaft 144 may further be securable once positioned so thatfurther rotation is restricted, such as by set screws as describedabove.

FIGS. 7 and 8 illustrate applications of the trackers 122 and trackers130, respectively. In both cases, two bone pins 122, 130 are affixed tothe bone 101. The bone pins 102 of trackers 122 and trackers 130 can beindividually placed into the bone spaced apart from each other. In thisway, the trackers 122 and trackers 130 can provide a greater distancebetween markers than with a conventional frame-based tracker.

During registration, the localizer images the target space including theobject to be tracked, for example, a patient's anatomy, and the trackersaffixed to the object. The trackers 122, 130 affixed to the patient'sanatomy may be detected by the navigation computer 26 or may be selectedby the user to be designated as a single tracking array. Oncedesignated, the relative positions of the four markers, distributedbetween the two trackers 122, 130, is stored in the navigation computer26 in a defined relationship. The rigidity of the bone, combined withthe rigid affixation of the trackers 122, 130 to the bone, and thedefined geometric relationship of the two markers on each pin 122, 130provides the navigation computer 26 with information to perform a rigidtransformation of the coordinate systems of the trackers to thecoordinate system of the localizer. In this way, the navigation computercan track the movement of the object in three dimensions by tracking themovement of the markers.

A further improvement is realized in the use of the trackers 122, 130 byeliminating the need for a checkpoint screw. Owing to the separate,rigid attachment of the two trackers 122, 130 and the definedrelationship between the two markers on the trackers 122, 130, eachtracker 122, 130 can act as a continuous checkpoint for the other of thetrackers commonly affixed. The navigation computer 26 can track thelocation of the trackers 122, 130 in order to track the location of theobject and can track the location of the trackers 122, 130 relative toeach other to ensure that the registration remains true.

To provide full tracking, the present disclosure described affixingmultiple tracker supports to the object to be tracked, with each trackersupport including multiple markers. A single tracker support, includinga single marker, may be used to provide some level of partial tracking.Small objects, including those expected to be subject only to smalldisplacements, may be registered with a single marker point to define atracking relationship relative to other registered objects. In oneexample, a single tracking support is registered to a patient's patellain order to provide partial tracking relative to the patient's femur F,tibia T, or both.

Referring now to FIG. 9, a method 200 of tracking an object is provided.Prior to a surgical operation, the target site of a patient's anatomy isprepared for surgery. As part of the preparation, one or more trackersmay be affixed to the patient's anatomy and registered to the navigationsystem 20. At step 202 of the method, a first tracker support isaffixed, for example, to a patient's femur. The first tracker supportincludes bone pin 102, which may be affixed by first making a smallincision in the patient's leg to expose the bone for the surgeon'saccess. Once the bone is exposed, the surgeon may employ a power drillor other suitable tool to screw the bone pin 102 into the bone. The bonepin 102 is provided with a feature 104 to engage with the drill, and anend feature 120 to facilitate screwing into the bone. At step 204, asecond tracker support is affixed to the object to be tracked at alocation spaced apart from the first tracker support. The second trackersupport includes a second bone pin 102 and is affixed in the same manneras the first.

At step 206, a plurality of fiducial markers are secured to the firsttracker support and the second tracker support. In one example, thisstep may be performed by supporting a plate 110 at each end on the firstand second tracker supports. The plate 110 may be provided with markers118 formed thereon or which are applied once the plate 110 has beensupported on the first and second tracker supports, such as through theapplication of adhesive-backed IR-reflective stickers. In a secondexample, the step of securing the fiducial markers to the trackersupports may be performed by attaching a sleeve 124 onto each of thefirst and second tracker supports. The sleeve initially may berotationally adjustable on the tracker support and, once positioned,secured into place to resist further rotation. In a further alternativeexample, this step of securing the fiducial markers to the trackersupports may be performed by sliding a collar 132 including a post 136onto each of the tracker supports and thereafter clamping a fiducialsupport 138 onto the post by a clamp 140. The fiducial support mayprovide for additional positional adjustments before being finallysecured in place.

At step 208, a marker arrangement is defined in the navigation computerto be associated with object to be tracked. The markers defining thearrangement are supported by tracker supports spaced apart from eachother on the object to be tracked. In one example, the tracker supportsare bone pins disposed at opposite portions of a bone and support anelongated plate 110 containing the markers 118. The navigation computer26 may recognize the pattern of markers on the plate 110, if, forexample, the plate 110 is pre-marked with the markers 118 in apre-defined arrangement. The navigation computer 26 may store a libraryof known marker arrangements and correlate the detected pattern to oneof the known arrangements. Alternatively, the operator may provide aninput, for example, through touchscreen 30, to select from among thelibrary of known marker arrangements, which the navigation computer 26will thereafter associate with the detected pattern of markers on theplate.

Defining the marker arrangement for tracking, at step 208, in anotherexample, does not require any pre-defined arrangement of markers to bestored in navigation computer 26. Rather, the operator may be creatingthe arrangement in the course of performing the actions of steps202-206. That is, the operator may be applying the markers 118 to theplate 110 in addition to securing the plate 110 to the tracker supports.Likewise, assembling the trackers 122 and trackers 130 creates a uniquearrangement of markers 128, 146 in the relationship between the firstand second tracker supports. Even though the relationship from onemarker 128, 146 to the other on each tracker 122, 130 is predefinedaccording to the manufactured geometry, the relationship between onetracker 122, 130 to the other tracker 122, 130 is not established untilthe tracker supports, have been affixed.

In an example application, where the navigation computer 26 does nothave a pre-defined arrangement of markers stored from which toreference, the operator may be presented with a prompt by the navigationcomputer to selectively designate which markers will define thearrangement for tracking the object. The operator may perform thisdesignation, in one example application, by using the navigation pointerP to touch off each tracker 122, 130 to designate its inclusion to thenavigation computer 26, which is then stored for subsequent use duringtracking. In another example application, the navigation computer 26will present the operator with the detected marker points, for example,displayed on screen 29, and allow the operator to select which of thedetected marker points will define the arrangement for tracking. Thismay be a similar action where the operator has applied markers to theplate 110.

At step 210, the defined marker arrangement is registered to the objectwithin the navigation computer 26 to allow the object to be trackedwithin the virtual environment according to the movements of the markerscomprising the defined arrangement. In a process similar to theconventional registration, preoperative images of the object to betracked are used to generate a virtual 3D representation or model of theobject. Using the pointer P or other registration process, the definedmarker arrangement of step 208 is associated with the object model inthe virtual environment of the navigation computer 26. Once registered,the navigation system 20 can provide precise information about theposition and orientation of the object by tracking the movement of thetracker.

Although described in separate examples, elements from the many examplesmay be combined, swapped, modified or employed based on the descriptionof other examples. For example, the bone pin 102 shown in FIGS. 4A and4B, supporting plate 110, may further support a spherical reflectorball-type marker as described in connection with the example illustratedin FIG. 5C. More generally, a single tracker support may includemultiple types of fiducials or markers. A tracker, such as 44 or 46 asin FIGS. 1-3, may be pre-marked with one or more markers, such as alaser-etched pattern that is recognizable by the navigation computer 26,and is thereafter further marked with additional, reflective stickers todifferentiate and define unique marker arrangements for each object tobe tracked. Moreover, the tracker may support both active and passivetrackers according to the needs or preferences of the operator.

Registration Recovery

As described above, the navigation system 20 is configured to monitorand detect whether any marker in the defined marker arrangement deviatesfrom the determined position relative to the other markers in thearrangement. Such changes may occur, for example, when one of thetracker supports becomes loose from the anatomy, is bent, or isotherwise displaced with respect to the anatomy. In such instances, theregistration may be lost.

However, the versatility and functionality of the tracker configurationsdescribed above advantageously provide for the ability to easily recoverlost registration. As used herein, registration recovery is the processby which markers of a displaced tracker can be returned to the positionand/or orientation of the original geometry of the markers that existedat the time of registration, thereby restoring the original registrationwithout repeating the registration process. This recovery feature isenabled by the dynamic configuration of separately affixed andadjustable trackers, which also serve as a continuous checkpoint foreach other. This registration recovery feature advantageously saves asignificant amount of time in the operating room because a completeregistration process need not be repeated when registration is lost.

The techniques herein provide for the navigation system 20 to identifyconditions wherein at least tracker or marker of the arrangement(defined according to any of the components or techniques above) isdisplaced relative to an original registered geometry of thearrangement. The navigation system 20 can then take actions to eitherautomatically correct this condition or guide a user to address thecondition.

The registration recovery process can begin, for example, when thenavigation system 20 automatically detects tracker displacement and lossof registration. If registration is lost, the navigation system 20 canautomatically notify the operator that registration has beencompromised.

Once registration is lost, there are different scenarios by whichregistration recovery can be initiated and performed. In one example,the navigation system 20 can further detect whether the error conditionthat occurred (e.g., the tracker displacement) is a candidate forregistration recovery. For example, the navigation system 20 canevaluate the magnitude and/or direction of the displacement with respectto an acceptable threshold or tolerance. The threshold may bepredetermined based on data from the original registration, possiblerange of motion of the tracker(s), the relationship between theseparately affixed trackers to each other or to the bone, or anycombination thereof. If the displacement satisfies the displacementthreshold, then the navigation system 20 can determine that registrationrecovery may be possible. The navigation system 20 can also evaluatetracker movement over time, e.g., to determine whether the tracker isstable enough for registration recovery. For example, the navigationsystem 20 can determine that the tracker is still stably affixed to thebone, but the marker is in a different position. Alternatively, thenavigation system 20 can determine that the tracker is too unstable forregistration recovery (e.g., when the bone pin is too unstable), therebyrequiring repeating of the full registration process. There maytechniques other than those described herein by which the navigationsystem 20 can detect loss of registration and optionally, whether thetracker condition is a candidate for registration recovery. Ifregistration recovery is possible, the navigation system 20 canautomatically notify the operator that registration recovery can beperformed.

In some instances, the operator is aware which bone pin, tracker, ormarker(s) was displaced from its position and/or orientation thatexisted at the time of registration. For example, the user may havewitnessed the displacement. In such instances, the navigation system 20can instruct registration recovery steps in conjunction input based onthe knowledge of operator. Alternatively or additionally, based on theoperator's knowledge that the error condition is recoverable, the usermay manually initiate the registration recovery process using inputdevices (e.g., display 30) of the navigation system 20. For example, theoperator can input to the navigation system 20 which tracker wasdisplaced thereby decreasing the time needed to identify the errorcondition and recover registration.

In either instance, once the registration recovery process is initiated,the navigation system 20 monitors the tracker pose during recovery andthe operator can manipulate the bone pin, tracker or marker(s) until thenavigation system 20 determines that the original geometry is restored.

Referring to FIG. 10, the error condition causing loss of registrationmay have occurred because movement occurring at a rotatable feature ofthe tracker. The rotatable feature can be the threaded connection, suchas the connection between the threaded mounting interface 108 of thebone pine 122 to the bone. Additionally or alternatively, the rotatablefeature can be the sleeve 124 of the tracker described above. Therotatable feature can be other features not described herein. Thenavigation system 20 can determine that a change in orientation oftracker about the axis of rotation is needed to recover registration andcan instruct or guide the operator to rotate the feature (as shown) toits original position to recover registration. To facilitate thisprocess, the navigation system 20 can provide information on thedisplay(s) 28, 29 to assist the operator in recovering the positionand/or orientation of the tracker or bone pin that existed at the timeof registration. In FIG. 10, the tracker has the configuration of thetracker 122 shown in FIGS. 5A and 7. The navigation system 20 can show acamera-based or virtual representation of the tracker and bone tofacilitate the correction. In this example, the navigation system 20specifically instructs the operator to rotate the tracker in a certaindirection and by a certain magnitude to reach the original geometry ofthe tracker configuration. The navigation system 20 monitors thebehavior of the tracker during this correction and will notify theoperator once registration is recovered. The guidance can includemessages, videos, animations showing the exact correction needed, or thelike. Other examples of guidance provided by the navigation system 20are contemplated.

In another example, and referring to FIG. 11, the error conditioncausing loss of registration may have occurred because movement of aflexible part of the tracker. For instance, the flexible stem 142 of thetracker 130 of the example of FIG. 8 may have inadvertently moved fromits original position. The navigation system 20 can determine that aflexing of the stem 142 is needed to move the tracker to its originalposition to recover registration. In this example, the navigation system20 shows a virtual representation of the tracker in the originalposition on the display(s) 28, 20. The operator can then flex the stem124 while observing the real-time monitored movement of the stem 124 andtracker on the display 28, 29 until the tracker is properly positionedin the original position. This type of guidance may be useful where thecorrection is difficult to explain with specific instructions (as inFIG. 10). Once the tracker reaches the original position, the navigationsystem 20 can notify the operator of a successful recovery. In oneexample, the virtual representation of the tracker in the originalposition may, for example, be highlighted to visually show the operatorthat the tracker position is properly restored. Other examples of suchnotifications may be through audible, haptic, textual and/or othervisual techniques.

In other instances, the operator may not be aware that the bone pin,tracker or marker(s) was displaced from its position and/or orientationthat existed at the time of registration. Again, the navigation system20 can automatically notify the operator that registration has beenlost. However, when the operator is unaware, he/she cannot provide inputto the navigation system 20 about which tracker was displaced. If thenavigation system 20 cannot automatically determine which tracker hasbeen displaced, there may be additional techniques that the navigationsystem 20 can utilize to identify which tracker is displaced.

In one example, once registration is lost, the navigation system 20 caninstruct the operator to use the navigation pointer P to digitize ananatomical landmark. The anatomical landmark can be one instructed bythe navigation system 20 or one that the operator selects using his/herjudgement. Since the pointer P is tracked, the selected landmark isregistered by the navigation system 20. The navigation system 20 thencomputes a distance between the anatomical landmark and each marker(e.g., any of the markers 118, 127, 128 or 146) of the tracker setup.From these distances, and the original geometry of the tracker setup,the navigation system 20 can then identify which tracker is displacedfrom the original geometry. Registration recovery can then be performedfor the identified tracker.

In a variation of the above example, the robotic manipulator 56 can beutilized to touch the anatomical landmark, instead of the navigationpointer P. More specifically, the robotic manipulator 56 can utilize theattached surgical instrument 22 or energy applicator EA to touch theanatomical landmark. The selected anatomical landmark can be registeredin different ways. In one example, the navigation system 20 tracks theposition of the instrument tracker 48 on the robot 56 and fuses thisdata with known relationship data between the instrument tracker 48 andthe instrument 22 and known relationship data between the instrument 22and a tool center point of the energy applicator EA. Additionally, oralternatively, the anatomical landmark can be determined based on thekinematic pose of the robotic manipulator 56 and the known relationshipbetween the instrument 22 and the tool center point of the energyapplicator EA. In either instance, the navigation system 20 can computethe distances between the selected anatomical landmark and the markersto determine which tracker is displaced from the original geometry, asdescribed above. Registration recovery can then be performed for theidentified tracker.

In another example, the pointer P can be utilized to digitize the bonepin at a region where a base of the bone pin contacts the bone. Oncethis point is registered by the navigation system 20, the navigationsystem 20 can compute a current distance between the digitized point andone or more markers of the tracker attached to the respective bone pin.These current distances can be compared with the original distancesbetween the markers and the base of the bone pin that existed at thetime of registration. If the navigation system 20 determines that thecurrent and original distances are different, the navigation system 20can determine that this tracker is the one that has experienced somedisplacement from the time of registration. The recovery process canthen be carried out with respect to the identified tracker. On the otherhand, if the navigation system 20 determines that the current andoriginal distances are identical, the navigation system 20 can determinethat this tracker has not experienced any displacement from the time ofregistration. Therefore, by process of elimination, the other tracker isidentified as the one having experienced displacement, and the recoveryprocess can then be carried out with respect to the other tracker.

Other examples to identify the error condition may be performed withoutthe need of supplemental devices, such as the pointer P or robot 56. Forinstance, the operator may determine that the anatomical joint issufficiently in the same position between the time registration wasactive and the time registration was lost. In this example, the operatorcan manually articulate the joint. The trackers are monitored by thenavigation system 20 during joint articulation. Since joint motion is arotational, each of the markers will move according to an arc radiusthat can be determined by the navigation system 20. The navigationsystem 20 can compare the arc radii of the markers with original arcradii data about the markers that was obtained at the time registrationwas active. The navigation system 20 can identify the marker for whichthe arc radius data is different from before and after registration waslost. The identified marker, therefore, must be part of the tracker thathas experienced displacement causing the error condition.

In another example, an IR reflective marker, such as a sticker, can berigidly placed on the anatomy or any other object rigidly attached tothe anatomy, such as an anatomy holder, anatomy wrapping, or the like.The navigation system 20 can determine the position of the IR reflectivemarker relative to each of the markers in the tracker setup. From thesedistances, and the original geometry of the tracker setup, thenavigation system 20 can then determine which tracker is displaced fromthe original geometry.

In yet another example, the navigation system 20 can utilize machinevision (e.g., by using video camera 41) to measure distances from eachmarker in the tracker setup to an anatomical landmark. From thesedistances, and the original geometry of the tracker setup, thenavigation system 20 can then determine which tracker is displaced fromthe original geometry.

The techniques described above identify which tracker was displaced whensuch information is unknown to the operator or not automaticallydeterminable by the navigation system 20. Once the navigation system 20identifies the displaced tracker, the navigation system 20 can thenproceed with guiding the operator through the registration recoveryprocess for the identified tracker according to any suitable method,such as those described with respect to FIGS. 10 and 11. The techniquesdescribed above can be utilized individually or in combination.

According to another technique, when one of the trackers is displaced,it may be possible for the navigation system 20 to maintain asub-optimal (less accurate) registration, instead of regarding theregistration as completely lost. Such situations are possible where eachtracker in the setup includes more than two markers. Such trackers canbe those from the tracker examples shown in FIGS. 4A and 4B, or any ofthe tracker examples of FIGS. 5-8 which are modified to include morethan two markers. Having three or more markers on each tracker providesthe navigation system 20 with both position and orientation informationeach the tracker. This enables each tracker to stand alone in preservingregistration. In such configurations, even if registration is partiallycompromised due to displacement of one tracker, the navigation system 20can maintain registration based on the remaining three or more markersof the originally located (non-displaced) tracker. This technique may beutilized as one option instead of repeating the full registrationprocess or performing registration recovery.

Although the accuracy of registration will likely be less than theaccuracy of registration based on both trackers, the navigation system20 can take measures to ensure that the registration is acceptable givencertain conditions. For example, the navigation system 20 can store ordetermine a registration accuracy tolerance required for steps of asurgical procedure. If one tracker is displaced, the navigation system20 can monitor what step in the surgical procedure is being performedand assess the registration accuracy tolerance for the given step. Ifthe tolerance is acceptable for the given step, the navigation system 20can proceed with registration based on the non-displaced tracker. Thenavigation system 20 can notify the operator, using any appropriatetechnique or message, that registration is currently based on onetracker or that registration accuracy has decreased. If the navigationsystem 20 determines that full registration accuracy is needed foranother step in the surgical procedure (e.g., a step requiring a highdegree of registration accuracy), the navigation system 20 can notifythe operator to repeat the full registration process or performregistration recovery, using any of the techniques described above. Inother examples, the registration accuracy based only on one tracker maybe sufficient for all steps of the surgical procedure such that theabove analysis is not required by the navigation system 20.

A further capability of the navigation system 20 is the ability torecover registration computationally, without the need to guide theoperator to mechanically manipulate the displaced tracker to theoriginal geometry of the tracker setup. In one example, a tracker may bedisplaced after registration yet still detectable by the navigationsystem 20. A less accurate registration can still be temporarilymaintained because of the non-displaced tracker remaining in itsoriginal state from the time of registration. While maintaining thisregistration, the navigation system 20 can determine the displaced poseof the tracker, i.e., because the displaced tracker is still detectable.The navigation system 20 can then compare the displaced pose of thetracker relative to the original pose of the tracker at the time ofregistration. From this comparison, the navigation system 20 can updatethe geometry of the tracker setup to account for the difference. Forexample, the original geometry of the tracker setup can be modified toaccount for the displaced pose of the tracker. Alternatively, thenavigation system 20 can generate a new geometry of the tracker setupthat replaces the old geometry. In either instance, the navigationsystem 20 is able to fully recover multiple tracker registration withoutlosing registration capability in the process. This process can beperformed computationally by the navigation system 20 without operatorknowledge or intervention in adjusting the displaced tracker.Furthermore, the navigation system 20 can provide notifications orstatus updates about this type of registration recovery to the user,according to any suitable technique.

Several examples have been discussed in the foregoing description.However, the examples discussed herein are not intended to be exhaustiveor limit the invention to any particular form. The terminology that hasbeen used is intended to be in the nature of words of description ratherthan of limitation. Many modifications and variations are possible inlight of the above teachings and the invention may be practicedotherwise than as specifically described.

1. A method for operating a navigation system comprising a first trackersupport affixed to a rigid object and a first plurality of trackableelements secured to the first tracker support, a second tracker supportaffixed to the rigid object and a second plurality of trackable elementssecured to the second tracker support, the first tracker support and thesecond tracker support adapted to be independently secured to the rigidobject and separated by a distance, the navigation system comprising alocalizer configured to track the first and second plurality oftrackable elements, the method comprising the navigation systemperforming the steps of: defining a tracking arrangement to be trackedbased on a combination of the first and second plurality of trackableelements; registering a geometry of the tracking arrangement relative tothe rigid object; tracking the rigid object by detecting the registeredgeometry of the tracking arrangement; identifying a condition wherein atleast one trackable element has been displaced relative to theregistered geometry; and generating a response to address the identifiedcondition.
 2. The method of claim 1, wherein the navigation systemgenerates the response to address the identified condition by furthergenerating visual guidance or instructions to assist a user inmanipulating the at least one displaced trackable element to return tothe registered geometry of the tracking arrangement.
 3. The method ofclaim 2, further comprising the navigation system: detecting that the atleast one displaced trackable element has returned to the registeredgeometry; and recovering registration of the tracking arrangementrelative to the rigid object after detecting that the at least onedisplaced trackable element has returned to the registered geometry. 4.The method of claim 1, further comprising the navigation systemevaluating an amount of displacement of the at least one trackableelement relative to a predetermined displacement threshold or toleranceto determine whether recovering registration of the registered geometryis possible by manipulating the at least one displaced trackable elementto return to the registered geometry of the tracking arrangement.
 5. Themethod of claim 4, further comprising the navigation system: determiningthat recovering registration is possible based on the evaluating theamount of displacement; and in response, generating visual guidance orinstructions to assist an user in manipulating the at least onedisplaced trackable element to return to the registered geometry of thetracking arrangement.
 6. The method of claim 4, further comprising thenavigation system: determining that recovering registration is notpossible based on the evaluating the amount of displacement; and inresponse, generating a notification indicating that recoveringregistration is not possible or that a complete re-registration isrequired.
 7. The method of claim 1, wherein the navigation systemfurther generates the response to address the identified condition by:detecting a state of the at least one displaced trackable element;defining an updated tracking arrangement to be tracked based on acombination of the first and second plurality of trackable elementsincluding the detected state of the at least one displaced trackableelement; and recovering registration of the tracking arrangementrelative to the rigid object by registering an updated geometry of thetracking arrangement relative to the rigid object.
 8. The method ofclaim 2, wherein the first tracker support and second tracker supportcomprise first and second bone pins, respectively, and furthercomprising at least one plate secured to the first and second bone pins,the at least one plate comprising at least one surface having at leastone of the first plurality and the second plurality of trackableelements secured to the at least one surface, wherein the at least onesurface is bendable to reconfigure positioning of the at least one ofthe first plurality and the second plurality of trackable elements, andwherein the navigation system generates the response to address theidentified condition by further: generating visual guidance orinstructions to assist the user in bending the at least one surface ofthe at least one plate to return the at least one displaced trackableelement to the registered geometry of the tracking arrangement.
 9. Themethod of claim 2, wherein the first tracker support and second trackersupport comprise first and second bone pins, respectively, and furthercomprising a first sleeve and a second sleeve disposed onto each of thefirst and second bone pins, respectively, the first sleeve comprisingthe first plurality of trackable elements secured thereto and the secondsleeve comprising the second plurality of trackable elements securedthereto, the first and second sleeves being rotationally adjustablerelative to the first and second bone pins, respectively, to reconfigurepositioning of the at least one of the first plurality and the secondplurality of trackable elements, and wherein the navigation systemgenerates the response to address the identified condition by further:generating visual guidance or instructions to assist the user inrotating the at least one of the first and second sleeves to return theat least one displaced trackable element to the registered geometry ofthe tracking arrangement.
 10. The method of claim 2, wherein the firsttracker support and second tracker support comprise first and secondbone pins, respectively, the first and second bone pins each having acollar and a post extending from the collar, and further comprising afirst fiducial support comprising the first plurality of trackableelements and configured to clamp to the post of the first bone pin and asecond fiducial support comprising the second plurality of trackableelements and configured to clamp to the post of the second bone pin, thefirst and second fiducial supports each comprising a stem that isconfigured to bend to reconfigure positioning of the at least one of thefirst plurality and the second plurality of trackable elements, andwherein the navigation system generates the response to address theidentified condition by further: generating visual guidance orinstructions to assist the user in bending the stem of at least one ofthe first and second fiducial supports to return the at least onedisplaced trackable element to the registered geometry of the trackingarrangement.
 11. The method of claim 1, wherein the first plurality oftrackable elements comprises two or fewer trackable elements and thesecond plurality of trackable elements comprises two or fewer trackableelements, and wherein the navigation system defines the trackingarrangement to be tracked based on a combination of the two or fewertrackable elements of the first plurality and the two or fewer trackableelements of the second plurality.
 12. A navigation system comprising: afirst tracker support affixed to a rigid object and a first plurality oftrackable elements secured to the first tracker support; a secondtracker support affixed to the rigid object and a second plurality oftrackable elements secured to the second tracker support; the firsttracker support and the second tracker support adapted to beindependently secured to the rigid object and separated by a distance; alocalizer configured to track the first and second plurality oftrackable elements; and at least one controller coupled to the localizerand being configured to: define a tracking arrangement to be trackedbased on a combination of the first and second plurality of trackableelements; register a geometry of the tracking arrangement relative tothe rigid object; track the rigid object by detecting the registeredgeometry of the tracking arrangement; identify a condition wherein atleast one trackable element has been displaced relative to theregistered geometry; and generate a response to address the identifiedcondition.
 13. The navigation system of claim 12, wherein the at leastone controller is configured to generate the response to address theidentified condition by further being configured to generate visualguidance or instructions to assist a user in manipulating the at leastone displaced trackable element to return to the registered geometry ofthe tracking arrangement.
 14. The navigation system of claim 13, whereinthe at least one controller is further configured to: detect that the atleast one displaced trackable element has returned to the registeredgeometry; and recover registration of the tracking arrangement relativeto the rigid object after detecting that the at least one displacedtrackable element has returned to the registered geometry.
 15. Thenavigation system of claim 12, wherein the at least one controller isfurther configured to: evaluate an amount of displacement of the atleast one trackable element relative to a predetermined displacementthreshold or tolerance; and determine whether recovering registration ofthe registered geometry is possible by manipulating the at least onedisplaced trackable element to return to the registered geometry of thetracking arrangement.
 16. The navigation system of claim 15, wherein theat least one controller is further configured to: determine thatrecovering registration is possible based on the evaluating the amountof displacement; and in response, generate visual guidance orinstructions to assist an user in manipulating the at least onedisplaced trackable element to return to the registered geometry of thetracking arrangement.
 17. The navigation system of claim 15, wherein theat least one controller is further configured to: determine thatrecovering registration is not possible based on the evaluating theamount of displacement; and in response, generate a notificationindicating that recovering registration is not possible or that acomplete re-registration is required.
 18. The navigation system of claim12, wherein the at least one controller generates the response toaddress the identified condition by further being configured to: detecta state of the at least one displaced trackable element; define anupdated tracking arrangement to be tracked based on a combination of thefirst and second plurality of trackable elements including the detectedstate of the at least one displaced trackable element; and recoverregistration of the tracking arrangement relative to the rigid object bybeing configured to register an updated geometry of the trackingarrangement relative to the rigid object.
 19. The navigation system ofclaim 13, wherein: the first tracker support and second tracker supportfurther comprise first and second bone pins, respectively, and furthercomprising at least one plate secured to the first and second bone pins,the at least one plate comprising at least one surface having at leastone of the first plurality and the second plurality of trackableelements secured to the at least one surface, wherein the at least onesurface is bendable to reconfigure positioning of the at least one ofthe first plurality and the second plurality of trackable elements; andwherein the at least one controller generates the response to addressthe identified condition by further being configured to generate visualguidance or instructions to assist the user in bending the at least onesurface of the at least one plate to return the at least one displacedtrackable element to the registered geometry of the trackingarrangement.
 20. The navigation system of claim 13, wherein: the firsttracker support and second tracker support further comprise first andsecond bone pins, respectively, and further comprising a first sleeveand a second sleeve disposed onto each of the first and second bonepins, respectively, the first sleeve comprising the first plurality oftrackable elements secured thereto and the second sleeve comprising thesecond plurality of trackable elements secured thereto, the first andsecond sleeves being rotationally adjustable relative to the first andsecond bone pins, respectively, to reconfigure positioning of the atleast one of the first plurality and the second plurality of trackableelements; and wherein the at least one controller generates the responseto address the identified condition by further being configured togenerate visual guidance or instructions to assist the user in rotatingthe at least one of the first and second sleeves to return the at leastone displaced trackable element to the registered geometry of thetracking arrangement.
 21. The navigation system of claim 13, wherein:the first tracker support and second tracker support further comprisefirst and second bone pins, respectively, the first and second bone pinseach having a collar and a post extending from the collar, and furthercomprising a first fiducial support comprising the first plurality oftrackable elements and configured to clamp to the post of the first bonepin and a second fiducial support comprising the second plurality oftrackable elements and configured to clamp to the post of the secondbone pin, the first and second fiducial supports each comprising a stemthat is configured to bend to reconfigure positioning of the at leastone of the first plurality and the second plurality of trackableelements; and wherein the at least one controller generates the responseto address the identified condition by further being configured togenerate visual guidance or instructions to assist the user in bendingthe stem of at least one of the first and second fiducial supports toreturn the at least one displaced trackable element to the registeredgeometry of the tracking arrangement.
 22. The navigation system of claim12, wherein: the first plurality of trackable elements comprises two orfewer trackable elements; the second plurality of trackable elementscomprises two or fewer trackable elements; and the at least onecontroller further defines the tracking arrangement to be tracked basedon a combination of the two or fewer trackable elements of the firstplurality and the two or fewer trackable elements of the secondplurality.